You can use the formula for the expansion of (1+x)n to compute the expansion of (a+bx)n for any constants a and b. In the case that n is not a non-negative integer, this expansion will only be valid when ∣x∣<ba.
Using the expansion of (1+x)n
The expansion formula tells you that:
(1+x)n=1+nx+2!n(n−1)x2+⋯+(rn)xr+…
This holds in general whenever n is a non-negative integer, as in these cases only finitely many binomial coefficients are nonzero, whereas when n is not a non-negative integer this produces an infinite series, and only holds whenever ∣x∣<1.
You can use this to derive the binomial expansion for (a+bx)n using this formula. If a=0, then (a+bx)n=bnxn, but otherwise:
This expansion is valid when −41x<1, i.e. when ∣x∣<4.
Therefore, the first four terms of the expansion of 12−3x1 are 63+483x+2563x2+614453x3+…, and the range of values of x for which this expansion holds is ∣x∣<4.
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Solving binomial problems
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Binomial expansion
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Binomial expansion: (a+bx)^n
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FAQs - Frequently Asked Questions
For what values of n is the expansion of (a+bx)^n an infinite series?
When n is not a non-negative integer.
For what range of values is the expansion of (a+bx)^n valid?
In the case that n is not a natural number, this expansion will only be valid when |x| < |a/b|.
How do I expand (a+bx)^n?
You can use the formula for the expansion of (1+x)^n to compute the expansion for (a+bx)^n for any constants a and b.